Arboriculture & Urban Forestry 40(3): May 2014 DISCUSSION Caliper increase and height increase—instead of caliper and height—were used as the primary com- parison metrics because this removed slight differ- ences among trees in initial size at planting. Appli- cation of fertilizer resulted in a mean of 6 mm (Ilex, Figure 3) or 8 mm (Quercus, Figure 6) increase in caliper (mean over 40 plots in 16 fertilizer treat- ments) over the control during the four-year pe- riod in nursery field soil. There was little evidence to suggest that Quercus caliper increased with either number of fertilizer applications or amount of fer- tilizer applied (rate); one application at the low rate (0.33 historical) provided the same caliper response as the high rate (1.3 historical) divided into four ap- plications (Figure 6). The implications on capital savings from fertilizer production, transport, ap- plication, and potential leaching into ground water are obvious. Two applications for Ilex were needed at three of the four rates (i.e., all but the 1.0 rate) in order to produce significant caliper increase compared to the non-fertilized control (Figure 3). Height increase data generally mirrored that of caliper (i.e., fertilizing over a four-year period had no impact on Ilex height increase). Lack of differences in height increase could have been due to nursery crews shearing trees to about the same height in the plots; however, crews were given instructions to shear as they would for a typical tree of that size. The 14 cm increase in Quercus height at the end of the four-year period at the 1.0 historical rate (3.64 m) compared to the 0.33 (3.50 m) rate came at three times the fer- tilizer cost (Figure 4). The potentially lower market value for the slightly shorter (3.64 m versus 3.50 m = 4%) finished crop with the same caliper could be compared with the savings from purchasing and applying one-third the amount of fertilizer to deter- mine which is the better management alternative. There were few consistent responses of either genus from more than one application at any fertil- izer rate. One application at the 0.33 rate, which was one-third of the grower’s historical rate, was enough to cause a small but significant growth response compared to the control. Increasing rate from 0.33, or dividing the rate equally among two, three, or four applications, resulted in little or no increase in growth. In a similar study on palms (Roystonea elata Bartr.) growing in a field nursery in south Florida, reducing N to half the grower’s historical rate had 183 no impact on growth of any measured parameter (Migliaccio et al. 2008). Gilman and Yeager (1990) and Gilman et al. (2000) concluded that laurel oaks (Quercus laurifolia Michx.), Japanese ligustrum (Ligustrum japonicum Thunb.), southern magno- lia (Magnolia grandiflora L.), and live oak (Quercus virginiana Mill.) in the same geographic region as the current study grew as well with or without fer- tilization. Broschat et al. (2008) suggested that not only are many tested dicots insensitive to the type of fertilizer they receive, but that under the soil and environmental conditions at many test sites, they may not benefit greatly from fertilization at all (sup- ported by Ponder et al. 1984; Robbins 2007; Watson 2010). Extensive root systems on established woody plants (Stout 1956; Watson and Himelick 1982) probably allow for a high capacity to scavenge min- erals and N needed for growth and development. Rose and Joyner (2003) suggest that growth response may occur primarily in poor urban soils and not in fertile soils typical in nurseries and in university test fields, where most studies have been performed; however, they present no evidence of this. Ingram et al. (1998) suggested 28 g N/m2 as adequate for many shade trees growing in Ken- tucky, U.S., nurseries; however, no application area was provided, making it impossible to calculate how much N was applied to the trees. This rate falls between the 20 g N/m2 duction, applied to a 0.65 m2 and the 40 g N/m2 (for the first two years of pro- circle around the tree) (for the third and fourth years) that resulted in the best growth for southern mag- nolia in a sandy field nursery in Florida (Gilman et al. 2000). Higher rates resulted in no more growth. Robbins (2007) showed no tree response, com- pared to non-fertilized controls, to a fairly low rate of 11 g N/m2 for Zelkova serrata (Thunb.) growing in an Arkansas, U.S., field nursery. Perhaps this rate was too low or the soil too fertile to provide a response in this soil type (not provided), or more likely the applied soil area (0.093 m2 of N because it was applied to a larger (0.84 m2 ), but at nine times the amount ) area, ) was very small resulting in little applied N (1 g N/tree or approxi- mately 0.5 g N/cm caliper). As evidence of a rela- tively small amount of N, Mathers (2012) applied the same rate (11 g N/m2 thus explaining the resulting tree response. This suggests that rate calculated as amount of applied N/tree or N/cm caliper may provide a more reliable ©2014 International Society of Arboriculture
May 2014
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